This invention relates to methods for preparing preforms useful in resin transfer molding processes and preforms prepared thereby.
Resin transfer molding is a molding process wherein a reinforcement material, such as fiberglass or other fiber reinforcement material, is positioned within a mold into which resin is subsequently introduced. It is a closed mold, low pressure process applicable to the fabrication of complex, high performance articles of both large and small size. The reinforcement material, hereinafter called "reinforcement fiber," is used to increase the strength of the resin several fold. For some applications involving complicated shapes, it is often desired to use a pre-shaped reinforcement fiber, also known as "preform", which has been coated with a small amount of resin to hold its shape during the molding process. Typically, the preform is positioned within a molding tool cavity and the molding tool is then closed. A feed line connects the closed molding tool cavity with a supply of liquid resin and the resin is pumped or otherwise transferred into the tool cavity where it impregnates and envelops the fiber reinforcement, and subsequently cures. The cured or semi-cured fiber-reinforced plastic product is then removed from the molding cavity.
Processes for making preforms are well known. Such processes employ conventional powder coating techniques to apply a tackifier onto the surface of a reinforcement fiber. In such processes, a tackifier, powder form, is distributed and fused on the surface of the reinforcement fiber. At the low tackifier concentrations utilized in preforming, it is difficult to obtain a uniform tackifier distribution with powder coating techniques due to the electrostatic charge generated on fine powders during the application operations. Uniform distribution of the tackifier is also difficult to achieve because of the high viscosity of the melted tackifier. Furthermore, the large particle size of the tackifier typically used in powder coating methods can cause compaction problems in the preform, making it difficult to obtain the preform thickness required for subsequent molding processes. Other known processes comprise dissolving the tackifier in a suitable solvent and then applying the tackifier solution onto the surface of the reinforcement fiber. However, the tackifier solution leaves a continuous film coating on the surface of the reinforcement fiber which becomes stiff upon evaporation of the solvent which, in turn, makes the coated reinforcement fiber difficult to make into a preform.
It would be desirable to provide a simple process for preparing preforms which contain a more uniformly distributed tackifier and which process does not have the disadvantages associated with the use of powder tackifiers and solution tackifiers.